Types of High-Rise Buildings Structural Systems

A multistory building higher than 21m or 21 to 29 floor buildings with unknown height described as high-rise structure. Various structural systems are available to be used in the construction of high rise building.

In this article, different types of high rise structural systems are presented.

Types of High-Rise Buildings Structural Systems

1. Braced frame structural system

• Braced frames are cantilevered vertical trusses resisting laterals loads primarily diagonal members that together with the girders, form the “web” of the vertical truss, with the columns acting as the “chords’’.
• Bracing members eliminate bending in beams and columns.

Fig.1:Different bracing types

• It is used in steel construction
• This system is suitable for multistory building in the low to mid height range.
• efficient and economical for enhancing the lateral stiffness and resistance of rigid frame system.
• This system permits the use of slender members in a building.
• An outstanding advantage of braced frame is that, it can be repetitive up the height of the building with obvious economy in design and fabrication.
• However, it might obstruct internal planning and the location of doors and windows. That is why it shall be incorporated internally along with lines of walls and partitions.

Fig.2:Braced frame structure

2. Rigid frame structural system

• In rigid frame structure, beams and columns are constructed monolithically to withstand moments imposed due to loads.
• The lateral stiffness of a rigid frame depends on the bending stiffness of the columns, girders and connections in-plane
• It is suitable for reinforced concrete buildings.
• It may be used in steel construction as well, but the connections will be costly.
• One of the advantages of rigid frames is the likelihood of planning and fitting of windows due to open rectangular arrangement.
• Members of rigid frame system withstand bending moment, shear force, and axial loads.
• 20 to 25 storey buildings can be constructed using rigid frame system.
• Advantages of rigid frame include ease of construction, labors can learn construction skills easily, construct rapidly, and can be designed economically.
• Maximum beam span is 12.2m and larger span beams would suffer lateral deflection.
• A disadvantage is that the self-weight is resisted by the action from rigid frames.
• Finally, Burj Al Khalifa which is the tallest structure in the world is constructed using rigid frame system.

Fig.3:Rigid frame structural system

3. Wall-frame system (dual system)

• It consists of wall and frame that interact horizontally to provide stronger and stiffer system.
• The walls are usually solid (not perforated by openings) and they can be found around the stairwells, elevator shafts, and/or at the perimeter of the building.
• The walls may have a positive effect on the performance of the frames such as by preventing a soft storey collapse.
• Wall-frame system suitable for buildings with storey number ranges from 40-60 storey which is greater than that of shear or rigid frame separately.
• braced frames and steel rigid frames provide similar advantages of horizontal interaction.

Fig.4:wall frame system

4. Shear wall system

• It is a continuous vertical wall constructed from reinforced concrete or masonry wall.
• Shear walls withstand both gravity and lateral loads, and it acts as narrow deep cantilever beam.
• Commonly, constructed as a core of buildings
• It is highly suitable for bracing tall buildings either reinforced concrete or steel structure. This because shear walls have substantial in plane stiffness and strength.
• Shear wall system is appropriate for hotel and residential buildings where the floor-by floor repetitive planning allows the walls to be vertically continuous.
• It may serve as excellent acoustic and fire insulators between rooms and apartments.
• shear wall structural system can be economical up to 35 stories building structure.
• Shear walls need not to be symmetrical in plan, but symmetry is preferred in order to avoid torsional effects.

Fig.5:shear walls system

5. Core and outrigger structural system

• Outrigger are rigid horizontal structures designed to improve building overturning stiffness and strength by connecting the core or spine to closely spaced outer columns
• The central core contains shear walls or braced frames.
• Outrigger systems functions by tying together two structural systems (core system and a perimeter system), and render the building to behave nearly as composite cantilever.
• The outriggers are in form of walls in reinforced concrete building and trusses in steel structures.
• Multilevel outrigger systems can provide up to five times the moment resistance of a single outrigger system.
• Practically, Outrigger systems used for buildings up to 70 stories. Nonetheless, it can be used for higher buildings.
• Not only does the outrigger system decline building deformations resulting from the overturning moments but also greater efficiency is achieved in resisting forces.

Fig.6:Outtrigger structure system

6. Infilled frame structural system

• Infilled frame structure system consists of beam and column framework that some of the bays infilled with masonry, reinforced concrete, or block walls.
• Infill walls can be part-height or completely fill the frame.
• The walls may or may not be connected to the formwork.
• Great in plan stiffness and strength of the walls prevent bending of beams and columns under horizontal loads. As a result, frame structural performance will be improved.
• During an earthquake, diagonal compression struts form in the infills so the structure behaves more like a Braced Frame rather than a Moment Frame.
• It can build up to 30 storey buildings.

Fig.7:Infilled frame structure system

7. Flat plate and flat slab structural system

• This system consists of slabs (flat or plate) connected to columns (without the use of beams).
• flat plate is a two-way reinforced concrete framing system utilizing a slab of uniform thickness, the simplest of structural shapes.
• The flat slab is a two-way reinforced structural system that includes either drop panels or column capitals at columns to resist heavier loads and thus permit longer spans.
• Lateral resistance depends on the flexural stiffness of the components and their connections, with the slab corresponding to the girder of the rigid frame.
• Suitable for building up to 25 stories.

Fig.8:Flat slab and flat plate structure system

8. Tube structural system

• This system consists of exterior columns and beams that create rigid frame, and interior part of the system which is simple frame designed to support gravity loads.
• The building behaves like equivalent hollow tube.
• It is substantially economic and need half of material required for the construction of ordinary framed buildings.
• Lateral loads are resisted by various connections, rigid or semi-rigid, supplemented where necessary by bracing and truss elements.
• It is used for the construction of buildings up to 60 storeys.
• Types of tube structure system include framed tube system (fig.9), trussed tube system (fig.10), bundled tube system (fig.11), and tube in tube system (fig.12).
• Trussed tube system is formed when external bracing added to make a structure stiffer. This structure type suitable for building up to 100 storeys.
• Bundled tube system consists of connected tubes and it withstand massive loads.
• A tube-in-tube system (hull core) is obtained, if the core is placed inside the tube frame structure.

Fig.9:Framed tube structure system

Fig.10:Trussed tube system

Fig.11:Bundled tube structure system

Fig.12:Tube in a tube system

9. Coupled wall system

• This system composed of two or more interconnected shear walls
• Shear walls connected at the floor levels by beam or stiff slabs.
• Stiffness of the whole system is far greater than that of its components.
• The effect of the shear-resistant connecting members is to cause the sets of walls to behave in their partly as a composite cantilever, bending about the common centroidal axis of the walls.
• The system is suitable for buildings up to 40 storey height.
• Since planer shear walls support loads in their plane only, walls in two orthogonal directions need to withstand lateral loads in two directions.

Fig.13:Coupled wall system

10. Hybrid structural system

• It is the combination of two or more of basic structural forms either by direct combination or by adopting different forms in different parts of the structure.
• Its lack of torsional stiffness requires that additional measures be taken, which resulted in one bay vertical exterior bracing and a number of levels of perimeter vierendeel “bandages”

Fig.14:Vierendeel frame

• It can be used for the buildings of as high as 300m.
• According to chines code (JGJ3-2002), hybrid system can be used for the construction of buildings with maximum 150m height in seismic regions.

Fig.15:Hybrid structure system

Determine Effect of Trench Excavation on Nearby Buildings by Rule of Thumb

Clearly, trench excavation affects nearby buildings and causing them to settle. The major factors that cause building settlement are soil relaxation and lowering groundwater. Both of these factors are triggered by the trench excavation.

This articles presents the determination of trench excavation influence on nearby buildings by rule of thumb.

Fig.1: Effect of trench excavation on nearby buildings

Trench excavation influence nearby buildings because of two reasons which are:

• Soil relaxation due to excavation
• Groundwater lowering because of excavation

Soil relaxation due to excavation

When a trench excavated close to a building, soil relaxation and subsequent settlement of the building is highly likely as shown in figure 1.

Fig.2: settlement or failure of building foundation due to trench excavation

Therefore, it is necessary to provide adequate support for the trench to prevent soil relaxation as illustrated in figure 2. Alternatively, excavate the trench away from the foundation which may not be a valid option all the time.

Fig.3:Support trench excavation to prevent soil movement

Moreover, a rule of thumb can be used to check whether the trench excavation influences the nearby building or not. Draw a line with 2H:1V from the bottom of the foundation as shown in figure 4. If the trench is within this line, then engineer shall expect soil relaxation and eventual settlement of building foundation.

Fig.4: The building is within the line, prevent this condition

Finally, if the soil at project site is very loose sandy soil, then draw the line with 3H:1V rather than 2H:1V, as illustrated in figure 5.

Fig.5: line of checking the trench excavation affect on nearby building in very loose sandy soil

Groundwater lowering because of excavation

By and large, groundwater moves and seeps into the excavation from the surrounding areas of the trench. This will lower the level of ground water in the trench vicinity area as illustrated in figure 6. Effective stresses increase as a result of groundwater lowering and consequently the foundation suffer settlement.

Lastly, lowering ground water increases effective stress in clay layer shown in figure 6, and greater effective stress causes foundation settlement.

Fig.6: Groundwater seeps into the excavation

Performance Levels of Buildings Against Earthquakes

Performance level of structures against earthquakes describes limiting damage condition that assumed to be satisfactory for a given building and a given ground motion.

Moreover, building damages, danger to life safety of occupants in the building due to the damage, and post-earthquake serviceability of the building describe and control the limiting damage condition.

Added to that, building performance level against earthquakes is a combination of the performance of both structural and nonstructural components.

Lastly, performance levels of building structures against earthquake will be presented in the following sections.

Fig.1:Performance levels of buildings against earthquakes

Performance levels of buildings against earthquakes are as follows:

• Operational performance level
• Immediate occupancy performance level
• Life safety performance level
• Collapse prevention performance level

Operational performance level

• This performance level associates with functionality of the structure. Generally, all systems important to normal operation are operational.
• Damage to the building is limited, so the overall damage is very light and hence immediate occupancy is not questionable.
• The structure does not experience permanent drift.
• The building retains original strength and stiffness considerably.
• facades, partitions, and ceilings as well as structural elements suffer minor cracking only.
• Nonstructural component damage is negligible.
• The structure requires minor repairing which can be done without important disruption to occupants.
• Finally, power and other utilities are available; possibly from standby sources.

Fig.2:operational building performance level, very light overall damage

Immediate occupancy performance level

• The structure experience light damages
• There is no permanent drift.
• The building retains original strength and stiffness substantially.
• Minor cracking of facades, partitions, and ceilings as well as structural elements.
• Elevators can be restarted.
• Fire protection operable.
• The building space and systems are anticipated to be fairly usable. however, equipment and contents are generally secure but may not operate due to mechanical failure or lack of utilities.
• Concrete frame experience minor hairline cracking, limited yielding at few locations, and no crashing (strain of concrete less than 0.003)
• Steel moment frames experience minor local yielding at few locations. No buckling, fracture, and observable distortion of members.
• Lastly, braces of Braced steel frame structure suffer minor yielding or distortion

Fig.3:Immediate occupancy building performance level, moderate overall damage

Life safety performance level

• This level intended to obtain a damage condition that presents a substantially low probability of danger to life safety. Whether the danger is due to structure damage or fallen of nonstructural components of a building.
• The building experiences moderate overall damage
• All stories of a structure retain some residual strength and stiffness left in.
• Gravity-loadbearing elements function.
• There will be no out of plane failure of walls or tipping of parapets.
• However, the structure undergoes some permanent drift.
• partitions suffer damage.
• Building may be beyond economical repair.
• Falling hazards mitigated but many architectural, mechanical, and electrical systems are damaged.
• Concrete frame beams damage extensively, shear cracking and cover spall off occur in ductile columns, and minor cracking develops in nonductile columns.
• Hinges create in steel moment frames. In addition to local buckling of some beams, serious joint distortion, and fracture of isolated moment connection. However, shear connection would remain sound and few elements might suffer partial fracture.
• Lastly, in braced frames, majority of bracing buckle or yield but do not fail entirely, and several connections may fail as well.

Fig.4:life safety occupancy building performance level, moderate overall damage

Collapse prevention performance level

• This level of building performance mainly relates to the vertical load carrying system and the structure need to stable under vertical loads only.
• Generally, the building damage is severe
• The structure retains little residual stiffness and strength.
• however, load bearing columns and walls function.
• The building suffers Large permanent drifts.
• Some exits blocked.
• Infills and unbraced parapets failed or at incipient failure.
• Building is near collapse.
• Nonstructural components damage extensively.
• In concrete frames, hinges and extensive cracking develop in ductile elements, nonductile columns experience splice failure and limited cracking, and short columns damage seriously.
• Beams and columns distort heavily in steel frames. Added to that, several moment connections fracture but shear connections remain intact.
• Finally, braces yield and buckle extensively in braced frames, and even many of them along with their connections could fail.